An anti-cross threading screw and a method of producing it are known from PCT Application WO 97/04241 corresponding to U.S. Pat. Nos. 5,730,566, 5,791,849, 5,997,231 and 6,162,001 as well as German Patent No. DE 696 10 838 T2. The known anti-cross threading screw includes a head, a shank and a threaded portion including a thread having a pitch, the threaded portion facing the free end of the screw. The thread and its pitch correspond to the inner thread of a corresponding nut. The threaded portion is designed to be continuous in an axial and a radial direction, and it includes a rounded thread crest in the region of its portion facing the free end of the screw. The rounded thread crest at least extends over one revolution of the thread, meaning about at least 360°. This part of the threaded portion being continuous in an axial and a radial direction serves as a self-centering portion. Due to the rounded shape and the outer diameter being decreased compared to a “usual” thread, the outer diameter of the screw during continued tightening starting from a misaligned position can be aligned with respect to the inner thread of the nut in a way to correct cross threading without elastic and plastic deformation of the thread. Preferably, the threaded portion of the “usual” thread as well as the rounded beginning portion are produced by one single special rolling tool including rolling portions corresponding to the two different portions of the thread.
Another anti-cross threading screw is known from European Patent Application No. EP 0 942 181 A2 corresponding to U.S. Pat. No. 6,158,938 . The screw includes a head, a shank and a threaded portion having a thread and a pitch, the threaded portion facing the free end of the screw. The threaded portion serves to attain a threaded connection with a corresponding inner thread of a component in a usual way. In the region of its free end, the screw includes a diminished centering portion (see FIGS. 2 and 6). The diminished centering portion may also include spaced apart cams instead of a thread being continuous in a radial direction, the cams being located directly adjacent to the remaining thread and in a way that they have the same pitch.
Another anti-cross threading screw is known from U.S. Pat. No. 6,796,761 B2. According a special embodiment illustrated in FIGS. 12 and 13, the screw includes two axially spaced apart threaded portions, namely a first threaded portion facing the head of the screw and a second threaded portion facing the free end of the screw. A smooth shank portion is arranged between the threaded portions. The shank portion has an outer diameter being identical to the outer diameter of the second threaded portion. The first outer diameter of the first thread of the first threaded portion is substantially greater than the second outer diameter of the second thread of the second threaded portion. The actual self-centering function is supplied by the first threaded portion including a conical portion extending over a plurality of rotations of the thread and having a rounded thread crest and rounded flanks. The thread crest of the second threaded portion facing the free end of the screw however has a normal design, meaning it is rather sharp-edged. According to the description in column 5, line 23-27, the pitches of the threads are identical. The following sentence mentions that the pitches may also be different from each other. However, this has no substantial effect on the functionality of the known screw since the thread of the second threaded portion is substantially smaller than the inner thread of the corresponding component to be connected to the screw such that there is no engagement between these threads. The inner thread of the component corresponds to the outer thread of the first threaded portion.
Further self-centering screws are also known from U.S. Pat. No. 6,516,650 B1 as well as U.S. Pat. No. 6,561,741 B2.
A screw for producing a threaded connection with a nut or with a different component including a corresponding inner thread is known from German Patent Application No. DE 36 01 389 A1 corresponding to U.S. Pat. No. 4,854,794 . To prevent the threaded connection from being unintentionally loosened, the thread of the screw has a different pitch than the tread of the corresponding component. In this way, during tightening of the connection in the sense of screwing the outer thread of the screw into the inner thread of the corresponding component, one produces an increasing frictional moment leading to a jamming effect resulting in the threaded connection being secured in the tightened position.
Another prior art document generally describing how screws are produced is the German technical book Bossard: “Handbuch der Verschraubungstechnik”, expert-verlag, pages 171-172, item 6.3.